Lyme disease was first described by Steere et al. in 1977 who reported an epidemic of arthritis in Lyme, Old Lyme and East Haddam, Connecticut. In 1982, Dr. Willy Burgdorfer discovered a new spirochete in the midgut of Ixodes dammini ticks (see Burgdorfer, W. A. et al., (1982) Science 216: 1317-1319). This spirochete was subsequently shown to elicit an immune response in infected rabbits which paralleled that in humans with Lyme disease, and is now known to be the etiological agent of the disease. The spirochete was subsequently named Borrelia burgdorferi.
While Ixodes ticks are the most commonly encountered vectors of Borrelia burgdorferi, the spirochetes have also been found in deerflies, horseflies and mosquitoes. Spirochetes enter an animal host when the animal is bitten by the vector. The B. burgdorferi spirochetes reside in various tissues of the host animal, where they may lead to long term infection.
Borrelia burgdorferi have recently been shown to possess a unique type of extra chromosomal DNA, linear plasmids, which range in length from 0.5 to 50 kb (Bergstrom, S. et al., (1991) Scand. J. Infect. Dis.--Suppl. 77: 102-107). These plasmids contain the genes encoding the two major outer surface proteins (Osp) expressed by B. burgdorferi, OspA and Osp B (Bergstrom et al., 1991). These proteins are believed to be major antigens involved in the immunity to B. burgdorferi infection. Schwan and Simpson (Schwan, T. G. and Simpson, W. J., (1991) Scand. J. Infect. Dis.--Suppl. 77: 94-101) report heterogeneity of the Osp A and B proteins amongst different B. burgdorferi isolates, as well as on the same isolate grown at different temperatures in vivo and studies at different stages during the infection of mice.
The initial stage of Lyme disease is characterized by an expanding skin lesion, erythema chronicum migrans (Asbrink, E. and Hovmark, A.,(1988) Ann. N.Y. Acad. Sci. 539: 4-15; Halperin, J. J., (1991) Scand. J. Infect. Dis.--Suppl. 77: 74-80). Subjects also frequently develop arthritis. However, in only a small percentage of these subjects is the arthritis chronic (Steere, A. C., (1991) Scand. J. Infect. Dis.--Suppl. 77: 51-54). Borrelia burgdorferi may also infect the heart muscle. Cardiac involvement in Lyme disease has mainly been reported as transitory. However, animal studies revealed that skeletal and heart muscles are regularly affected and that the spirochetes appeared to be located within the muscle fibers. This observation suggests that the spirochetes are able to maintain long-term survival in hosts and may thus be able to cause chronic heart complications (Stanek, G. et al. (1991) Scand J. Infect. Dis.--Suppl. 77: 85-87).
B. burgdorferi also infects the nervous system in a high percentage of cases, leading to a wide range of acute, chronic and progressive central and peripheral nervous system disorders (Reik, L. et al. (1991) Ann. N.Y. Acad. Sci. 539: 1-3). Published reports have indicated that European populations afflicted with the disease experience dramatic clinical phenomena of neurological disorder, while North American patients develop milder forms of nervous system involvement. (Halperin, 1991; Halperin, J. J. et al., (1988) Ann. N.Y. Acad. Sci. 539: 24-34). However, Halperin (1991) report that it is becoming increasingly clear that at least as far as nervous system involvement is concerned, the same range of neurological phenomena occur in both populations. Diagnosis of Lyme disease depends upon a combination of the recognition of the clinical characteristics presented and also of the probability of exposure in endemically infected areas. However, the development of arthritis is often attributed to other causes and not correlated with a spirochete infection. The neurological symptoms which may be the result of B. burgdorferi infection can mimic a variety of other neurological conditions (Reik et al., 1988). Adding to these diagnostic complications is the difficulty of detecting the spirochetes in affected tissues.
Borrelia infection has been combatted with antibiotics, e.g., tetracycline, penicillin, amoxycilin, doxycilin, erythromycin and phenoxymethylpenicillin (Neu, H., (1988) Ann. N.Y. Acad. Sci., 539: 314-316; Skoldenberg, B. et al. (1988) Ann. N.Y. Acad. Sci. 539: 317-323; Weber, K. et al. (1988) Ann. N.Y. Acad. Sci. 539: 325-345; Luft., B. J. et al. (1988) Ann. N.Y. Acad. Sci, 539: 352-361). Ceftriaxone, and chemically similar compounds, have also been useful as chemotherapeutic agents (Neu, 1988). However, the establishment of protocols for effective chemical treatment of Lyme disease has been hampered by the lack of data with which an appropriate time period for treatment could be established. Additionally, there has been a lack of studies of the effectiveness of drugs in human patients. Further complicating the therapeutic picture is the need to establish and maintain sufficiently high tissue concentrations of antibiotics to combat chronic Borrelia infection (Neu, 1988; Skoldenberg et al., 1988).
Given this difficulty in the detection and treatment of Lyme disease, as well as the impracticability of managing the spread of spirochete vectors, there has been a recognized need for a vaccine capable of immunizing susceptible animals and humans against Borrelia burgdorferi infection. Vaccines have been studied in hamster (Johnson, R. C. et al. (1988) Ann. N.Y. Acad. Sci, 539: 258-263) and rat (Barthold, S. W. et al. (1988) Ann. N.Y. Acad. Sci. 539: 264-273) models. A whole cell Borrelia burgdorferi bacterin for use in domestic animals has been developed (U.S. Pat. No. 4,721,617). Vaccines based upon the B. burgdorferi Osp A and/or B proteins have also been developed. However, these whole cell and subunit vaccines contain, or are derived from, only one B. burgdorferi isolate. By contrast, the bacterin of this invention contains, or is derived from, at least two non-crossprotective B. burgdorferi isolates. The bacterin of this invention therefore will provide immune protection against two different types of B. burgdorferi isolate, while previously described bacterins provide protection against only one type of isolate. Accordingly, the bacterin provided by this invention will be more useful to vaccinate animals against Lyme disease.